During the pulping of the chips lignin and other substances in the chips are dissolved into the pulping liquid.
In order to recover the pulping chemicals (as well as to produce energy) the black liquor is concentrated and then sent to a recovery boiler. The concentrated black liquor is combusted in the recovery boiler and heat is extracted from the hot flue gases. The sodium and sulfur compounds are recovered as sodium carbonate and sodium sulfide. The sodium carbonate and sodium sulfide exit the recovery boiler in a molten state and are dissolved in a water solution (weak white liquor) thus forming green liquor.
The green liquor formed also contains small amounts of solid material, known as green liquor dregs or green liquor sludge. Because of the content of dregs in the green liquor, the green liquor needs to be clarified with respect to these dregs. One way to clarify the green liquor is by sedimentation another way is by filtration.
The chemical recovery is very often a bottleneck in the production of cellulose pulp and when the mills raise the production there is a need of raising the capacity of the chemical recovery process. Building larger apparatus for sedimentation of the green liquor is, however, a costly and complicated choice since the diameter the tank becomes large. Generally, clarification by sedimentation results in regenerated cooking chemicals with a high content of sludge. It is of great importance to keep the sludge content a low level in the cooking chemicals, otherwise the sludge will cause scaling and plugging in the pulping plant and the evaporation plant, which leads to unwished and unplanned, costly operation stops.
A general problem with filtration of green liquor is the low filtering capacity because of the poor filterability of the green liquor. The dregs in the green liquor form a dense cake on the filtering layer and blind the filtering medium thus lowering the filtering capacity even more. The filter cake must be removed and thereby causing production stops leading to lowered productivity and higher operating costs.
The document U.S. Pat. No. 5,3618,443 relates to clarification of green liquor by falling-film filtration. The filtering material is made of textile cloth. The apparatus described comprises a pressurized vessel in which several filter elements are mounted in a vertical position, or generally vertical position, and the liquid to be filtered flows due to the gravitational force along the filtering layers on the outside of the filter elements. Due to pressure difference, caused by pressurized gas, between outer and inner surfaces of the filter elements, the filtrate penetrates the filter surface from the outer side to the inner side of the filter surface of the elements and reaches the filtrate channel surrounded by the filtering layers.
A problem with prior art technique described above is the low filtering efficiency leading to large equipments and with high investment costs and a relative high energy consumption.
In order to solve the problems connected with clarification of green liquor, a filtering technique using a new kind of filter has been investigated. The filtering technique, hereinafter called cross-flow filtration, is described in a final thesis at the Royal Institute of Technology, Stockholm, by Fredrik Broström, 2007, TRITA-CHE-report 2007:66 ISSN 1654-1081.
The cross flow-filter comprises a long tubular ceramic membrane element perforated with several channels in its length direction. The green liquor is transported into the channels and while flowing through the channels the filtrate passes through the porous membrane channel walls and flows in a radial direction from the inside to the outside of the channels. The pore size of the ceramic membrane used was 45 μm.
However, in the thesis work the green liquor filtrate flow decreased rapidly, within 1 day the flow was ⅙ of the initial flow. This should according to the author be solved by controlling the inlet flow to the cross-flow filter.